The present invention is related to an improved polymerization method for preparing solid electrolytic capacitors. More specifically, the present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. Even more specifically, the present invention is related to a capacitor comprising interlayers wherein adjacent interlayers, particularly of the cathode, are cross-linked to each other thereby providing an improved cathode as indicated by improved ESR stability.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal preferably serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover up to all of the surfaces of the anode and to serve as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7,8,8 tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers all dielectric surfaces and is in direct intimate contact with the dielectric. In addition to the solid electrolyte, the cathodic layer of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a cathode conductive layer which may be a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; and a conductive adhesive layer such as silver filled adhesive. The layers including the solid cathode electrolyte, conductive adhesive and layers there between are referred to collectively herein as the cathode layer which typically includes multiple interlayers designed to allow adhesion on one face to the dielectric and on the other face to the cathode lead. A highly conductive metal lead frame is often used as a cathode lead for negative termination. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or by dipping, spraying, or printing of pre-polymerized dispersions.
The carbon layer serves as a chemical barrier between the solid electrolyte and the silver layer. Critical properties of the layer include adhesion to the underlying layer, wetting of the underlying layer, uniform coverage, penetration into the underlying layer, bulk conductivity, interfacial resistance, compatibility with the silver layer, buildup, and mechanical properties.
The cathodic conductive layer, which is preferably a silver layer, serves to conduct current from the lead frame to the cathode and around the cathode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and acceptable mechanical properties.
Today, almost all electronic components are mounted to the surface of circuit boards by means of infra-red (IR) or convection heating of both the board and the components to temperatures sufficient to reflow the solder paste applied between copper pads on the circuit board and the solderable terminations of the surface mount technology (SMT) components. A consequence of surface-mount technology is that each SMT component on the circuit board is exposed to soldering temperatures that commonly dwell above 180° C. for close to a minute, typically exceeding 230° C., and often peaking above 250° C. If the materials used in the construction of capacitors are vulnerable to such high temperatures, it is not unusual to see significant positive shifts in ESR which lead to negative shifts in circuit performance. SMT reflow soldering is a significant driving force behind the need for capacitors having temperature-stable ESR.
Equivalent Series Resistance (ESR) stability of the capacitors requires that the interface between the cathode layer, cathodic conductive layers, conductive adhesive, and leadframe have good mechanical integrity during thermo mechanical stresses. Solid electrolytic capacitors are subject to various thermomechanical stresses during assembly, molding, board mount reflow etc. During board mount the capacitors are subjected to temperatures above 250° C. These elevated temperatures create stresses in the interfaces due to coefficient of thermal expansion (CTE) mismatches between the interfaces. The resultant stress causes mechanical weakening of the interfaces. In some cases this mechanical weakening causes delamination. Any physical separation between the interfaces causes increases in electrical resistance between the interfaces and thus an increased ESR in the finished capacitor.
U.S. Pat. No. 6,304,427, which is incorporated herein by reference, teaches a method for improving ESR stability of capacitors. The combination of materials described therein offers some ESR stability, however, the method still allows an ESR rise of a few milliohms during board mount conditions.
Hahn et al., in U.S. Pat. No. 6,072,694, which is incorporated herein by reference, disclose an electrolytic capacitor whose adhesion of a conducting polymer film to an oxidized porous pellet anode is improved by the incorporation of a silane coupling agent in the polymer impregnating solution, in order to improve leakage and dissipation factor thereof. US Patent Publ. No. 2005/0162815, which is incorporated herein by reference, claims to provide an improved solid electrolytic capacitor by providing a coupling layer between a conducting polymer layer and dielectric layer, capable of bonding to both the conducting polymer layer and dielectric layer by covalent bonding, improving the adhesion and preventing voids from forming there between, thereby improving the electrical performance and reliability of the solid electrolytic capacitor
In the capacitor described in U.S. Pat. No. 7,489,498, which is incorporated herein by reference, the conductive adhesive layer and the cathode terminal are connected through the organic silane layer, as a chemical bond between the organic silane layer and the conductive adhesive layer is formed. Inventors claim this enhances the adhesive property of the conductive adhesive layer and the electrode thereby improving ESR.
In U.S. Pat. No. 7,800,887, which is incorporated herein by reference, ESR is improved by forming an intermediate layer containing organic silane on the first electrolyte layer and a step of forming a second electrolyte layer containing a conductive polymer on the intermediate layer.
In US Pat. Publ. No. 2010/0136222, which is incorporated herein by reference, a silane compound is added to the conductive polymer solution. The inventors claim the silane compounds strengthen the binding of a conductive polymer chain by a cross-linking effect.
US Pat. Publ. No. 2011/0026191, which is incorporated herein by reference, teaches a method of improving the ESR stability of capacitors by providing an insulative adhesion enhancing layer with the cathode layers. Although this method gives excellent ESR stability in many solid electrolytic capacitors, this method is not universally applicable for capacitors with various types of dielectric, cathode materials, and assembly process.
In U.S. Pat. No. 6,987,663, which is incorporated herein by reference, a polymeric cathode outer layer is generated by applying a dispersion comprising particles of a conductive polymer, such as polyethylene dioxythiophene:polystyrene sulfonic acid (PEDT:PSSA) and a binder. However, this method has the drawback that the ESR rises markedly under a thermal load, such as is produced for example during soldering of the capacitors. In US Patent Publ. No. US 2011/0019339, which is incorporated herein by reference, the inventors claim that ESR stability can be improved by applying a polymeric intermediate layer between the solid electrolyte and polymeric outer layer by means of dispersions comprising particles of a conductive polypyrrole and/or polyaniline and/or polythiophene, in particular of a conductive polythiophene, having an average particle diameter in the range of 1-60 nm, and a binder.
Solid electrolytic capacitors containing water dispersible conductive polymer dispersion coating such as PEDT:PSSA exhibits an unusually high ESR shift on exposure to SMT conditions. In U.S. Pat. No. 7,379,290, which is incorporated herein by reference, the inventors attribute the ESR instability of PSSA containing conductive polymer dispersion to undoping of PSSA. The inventors have found that undoping of PSSA can be suppressed by adding an additive naphthalene sulfonic acid.
Thus there is a need for a process for solid electrolytic capacitors with improved ESR and ESR stability. A particular need is for capacitor parts to have stable ESR during surface mount temperatures.